1. Field of the Invention
The present invention relates to a zoom lens suitable for a TV camera, video camera, and the like.
2. Related Background Art
As an autofocus (AF) system in a video camera or the like, a system called “hill-climbing AF” which obtains the distance to a principal object and drives a focus unit to an appropriate position is widely used.
In this system, the direction of the best imaging position is detected from a change in a video signal by using some of the lens units of an image pickup system for imaging, and the extension direction of a focusing lens is calculated. A merit of this system is that no optical system need be newly used for distance measurement.
In addition, in this system, in order to check whether the best imaging position is located before or after the image pickup surface (CCD surface or film surface), a partial optical system of the image pickup system is slightly amplitude-driven (to be referred to as wobbled hereinafter) in the optical axis direction. A focus unit is driven to set the best focus on the image pickup surface on the basis of the detected signal.
At this time, since part of the optical system is wobbled, the imaging magnification greatly changes to result in poor screen display unless lens units and the like are properly arranged.
As a method of reducing a change in imaging magnification, the method disclosed in Japanese Patent No. 2744336 is available. This method is associated with a rear focus zoom lens which has the first unit having a positive refractive power, the second unit having a negative refractive power for magnification changing operation, the third unit having a positive refractive power, and the fourth unit having a positive refractive power and serving to correct an image plane fluctuation with a change in magnification, and is designed to perform focusing by using the fourth unit.
A change in imaging magnification accompanying wobbling can be expressed by a paraxial trace. Several conditions for suppressing a change in imaging magnification accompanying wobbling small are described in U.S. Pat. No. 5,138,492.
These conditions are, however, insufficient to be applied to an image pickup system which has a wobbling lens unit placed in a relay lens unit located closer to the image side than the stop and allows insertion/removal of an extender for shifting the focal length to the telephoto side.
More specifically, the conditions described in the above reference are effective for a method of reducing a change in imaging magnification in a state where an optical arrangement behind the stop is fixed, but are not sufficient to be applied to a case wherein the arrangement of an optical system is changed upon insertion/removal of an extender or the like and the incident angle of an off-axis principal ray on a wobbling lens unit, in particular, changes.